1. Field of the Invention
The present invention relates to an image forming apparatus and a process cartridge.
2. Description of the Related Art
In an image forming apparatus, such as a printer, a copier, or a fax machine, an electrophotographic process is used to form latent images on an image carrier, a developing agent powder (referred to as a “toner” below) is attached to the latent images to develop the latent images into visible toner images. These toner images are transferred to a recording medium (including intermediate transferring members), and target images are formed.
FIG. 19 illustrates an example of a developing device for attaching the toner to the electrostatic latent images formed on the image carrier in the aforementioned image forming apparatus. As illustrated in FIG. 19, a developing roller 1002, to which a DC bias voltage is applied by a developing bias power supply 1003, is controlled to rotate at a speed V3 and is brought into contact with an image carrier 1001 rotating at a speed V1, and the toner particles, which are held on the developing roller 1002 by carriers C, are transferred to the latent images for developing. This is a so-called “two-composition contacting developing device”.
FIG. 20 illustrates another example of the developing device for attaching the toner to the electrostatic latent images on the image carrier.
As illustrated in FIG. 20, a developing roller 1004, to which a DC bias voltage is applied by a developing bias power supply 1005, is brought into contact with an image carrier 1001, and the toner, which is held on the developing roller 1004, is transferred to the latent images for developing. This is a so-called “one composition contacting developing device”.
There are still other methods of attaching the toner.
FIG. 21 illustrates still another example of applying the toner to the electrostatic latent images on the image carrier.
As illustrated in FIG. 21, a toner cloud is formed near the surface of an image carrier 1001, and the toner T in the form of cloud is attached to the latent images on the image carrier. This is a so-called “toner cloud method”.
FIG. 22 illustrates yet another example of attaching the toner to the electrostatic latent images on the image carrier.
As illustrated in FIG. 22, a DC bias voltage and an AC bias voltage are applied to the developing roller 1011 by a DC bias power supply 1012 and an AC bias power supply 1013, respectively, and an alternating electric field is formed between the image carrier 1001 and the developing roller 1011, thereby, the toner T held on the developing roller 1011 is jumped between the image carrier 1001 and the developing roller 1011, and thus the toner T is attached on the latent images for developing. This is a so-called “AC bias method”.
The inventors of the present invention have proposed a developing device used in the above image forming apparatus. The developing device has an electrostatic transportation device for transporting a developing agent and attaching the developing agent to the image carrier. The electrostatic transportation device includes a transportation substrate in which plural transportation electrodes are arranged on a supporting substrate, these transportation electrodes are repeatedly arranged along the direction substantially perpendicular to the moving direction, and are substantially in parallel to each other at predetermined intervals. These transportation electrodes generate an electrostatic force to move the developing agent. Reference can be made to Japanese Laid-Open Patent Application No. 2002-341656 and Japanese Laid-Open Patent Application No. 2002-307740 (referred to hereinafter as reference 1 and reference 2, respectively).
On the other hand, organic photo-conductors are presently widely used as image carriers, because the organic photo-conductors have high photo sensitivity and are low in price. The surface friction coefficient of an organic photo-conductor is determined by material properties of the organic photo-conductor, and generally it is in a range from 0.3 to 0.6. When transferring the toner to a piece of paper or an intermediate transferring member, if the toner adheres to the photo-conductor due to a non-electrostatic adhesive force, the toner cannot be transferred by a transfer electric field; as a result, transfer efficiency is lowered and image quality degrades because of insufficient transfer.
To solve this problem, a lubricant agent is introduced to reduce the non-electrostatic adhesive force, and in turn to reduce the surface friction coefficient. Reference can be made to Japanese Laid-Open Patent Application No. 11-288194 and Japanese Laid-Open Patent Application No. 2001-34048 (referred to as reference 3 and reference 4, respectively, hereinafter).
When reducing the surface friction coefficient of the image carrier as described in reference 3 and reference 4, however, although the problem of insufficient transfer is solved, the developing performance declines and images of desired density cannot be formed when using the common two-composition contacting developing device or one-composition contacting developing device.
It is thought that this problem is attributed to reduction of the non-electrostatic adhesive force imposed on the toner on the image carrier, which counteracts magnetic particle chains occurring in development or a mechanical scavenging effect (scraping effect) in a developing sleeve, and thus the toner falls off easily. As a result, by just reducing the surface friction coefficient of the image carrier, although transfer efficiency is improved, image quality degrades.
When the developing device employing the aforesaid toner cloud is used, the same problem occurs in that the toner adhering to the image carrier falls off easily. Hence, similarly, by just reducing the surface friction coefficient of the image carrier, although transfer efficiency is improved, image quality degrades.
When the developing device using the aforesaid AC bias is used, although the mechanical scavenging effect does not occur, the so-called electrical scavenging effect occurs; similarly, by just reducing the surface friction coefficient of the image carrier, although transfer efficiency is improved, image quality degrades.
As described in reference 4, the above problem can be alleviated more or less by setting the application time of the AC bias when attaching the toner to the image carrier long compared with the application time of the AC bias when removing the toner from the image carrier. However, because the AC bias is applied at the same time, the toner is also attached to the non-image portion of the latent image, which is not to be developed into an image, hence, the non-image portion is stained with the toner, that is, so-called scumming occurs.
Further, because of the scavenging effect, even when the surface friction coefficient of the image carrier is reduced, in practice, it is difficult to reduce the surface friction coefficient uniformly, and the surface friction coefficient changes with time. Especially, when the surface friction coefficient is less than 0.2, stripe-like unevenness occurs in the obtained image because of the non-uniform developing ability. For this reason, the surface friction coefficient has to be set in a range that does not cause the non-uniformity of the developing ability, and cannot be used in the range from 0.3 to 0.6. Therefore, transfer efficiency and cleaning efficiency cannot be improved sufficiently.
Recently, it is proposed to use a spherical toner produced by polymerization as the developing agent in order to improve transfer efficiency and cleaning efficiency. However, because the adhesive force between the spherical toner and the image carrier is even weaker, if the surface friction coefficient of the image carrier is reduced when the aforesaid toner cloud developing device or AC bias developing device is used, insufficient transfer occurs more easily.